1. Field of the Invention
This invention relates to a voltage dividing circuit and more particularly to a voltage dividing circuit capable of freely dividing a voltage as required and suitably used for a digital-analog converter or analog-digital converter, for example.
2. Description of the Related Art
In general, a voltage dividing circuit includes a plurality of resistors connected in series. The resistors are serially connected between terminals across which a voltage to be divided is applied. A desired divided voltage is derived from one of connection nodes between the resistors by adequately setting the number of the resistors and dividing each of the resistors into small resistance segments. The voltage can be divided with a small voltage step by increasing the number of the resistors and dividing each of the resistors into small resistance segments. However, there occurs a defect that the number of voltage divisions and the voltage step are previously determined by the number of the resistors and the resistances thereof. When each of the resistor is divided into small resistance segments, the pattern area may be increased and a multi-layered wiring will be required. Further, when the voltage is divided with a small voltage step, the output voltage level tends to be influenced by variation in the resistance caused in the manufacturing process of the resistors and will cause noises if the voltage dividing circuit is used in a D/A converter.
As a voltage dividing circuit capable of permitting selection of the number of voltage divisions, the circuit construction shown in FIG. 1 is known, for example. The voltage dividing circuit includes n resistors R1 to Rn, switches SW1 to SWn and a control circuit 11. The resistors R1 to Rn are serially connected between a terminal 12-1 to which a power source potential V.sub.DD is applied and a terminal 12-2 to which a ground potential V.sub.SS is applied (a group of series-connected resistors is called a "resistor string"). The switch SW1 is connected between one end N1 of the resistor R1 and an output terminal 13. Each of the switches SW2 to SWn is connected between a corresponding one of connection nodes N2 to Nn of the resistors R1 to Rn and the output terminal 13. The switching positions or ON/OFF positions of the switches SW1 to SWn are selectively controlled by an output signal of the control circuit 11. A voltage derived by dividing the voltage applied between the terminals 12-1 and 12-2 by a number selected according to the ON/OF positions of the switches SW1 to SWn is output from the output terminal 13. When the resistances of the resistors R1 to Rn are set equal to one another and the switches SW1 to SWn are sequentially turned on one at a time, one of output voltages V.sub.OUT corresponding to 1/n, 2/n, - - -, and (n-1)/n times a difference between the ground potential V.sub.SS and power source potential V.sub.DD is sequentially obtained. That is, the output voltage V.sub.OUT is a voltage derived by multiplying the voltage applied between the terminals 12-1 and 12-2 by X/n (X=1, 2, 3, - - -, or n-1).
In the circuit construction shown in FIG. 1, a voltage derived by selectively dividing the voltage applied between the terminals 12-1 and 12-2 by use of the switches SW1 to SWn can be output. However, since the resistance of the resistors R1 to Rn are previously determined at the stage of design, an output voltage V.sub.OUT having a desired voltage ratio which is not determined at the design stage cannot be obtained after the circuit is formed. The voltage dividing circuit of FIG. 1 is disadvantageous in another aspect. If each resistor is divided into more segments, the output voltage of the circuit tends to change due to the difference in resistance among the resistors, which has inevitably occurred during the manufacturing of these resistors, and also due to the fluctuation in the power-source voltage. A change in the output voltage, if any, will lower the resolution of the voltage dividing circuit. Instead of dividing each resistor into more segments, more resistors can be used to form the voltage dividing circuit. Obviously, this method increases the size of the voltage dividing circuit and, hence, the manufacturing cost thereof.